1. Field of the Invention
The present invention generally relates to cardiac pacemakers, and more particularly, pertains to cardiac pacemakers of the type which measure physiologic or metabolic requirements and vary the rate of the pacemaker in accordance therewith.
2. Description of the Prior Art
Early cardiac pacemakers provided a fixed-rate stimulation pulse generator that could be reset on demand by sensed atrial and/or ventricular depolarizations. Modern pacemakers include complex stimulation pulse generators, sense amplifiers and leads which can be configured or programmed to operate in single or dual chamber modes of operation, delivering pacing stimuli to the atrium and/or ventricle at fixed rates or rates that vary between an upper rate limit and a lower rate limit.
In recent years, single and dual chamber pacemakers have been developed which measure rate control parameters (RCP's) which are directly or indirectly related to metabolic requirements (e.g., demand for oxygenated blood) and vary the pacing rate in response to such measured RCP's. Such RCP's include, for example, physical activity of the body, right ventricular blood pressure and the change of right ventricular blood pressure over time, venous blood temperature, venous blood oxygen saturation, respiration rate, minute ventilation, and various pre and post-systolic time intervals measured by impedance or pressure sensing within the right ventricle of the heart. Such RCP-measuring, sensor-driven pacemakers have been developed for the purpose of restoring rate response to exercise in patients lacking the ability to increase rate adequately by exertion.
In general, a rate responsive pacemaker includes a sensor which produces a sensor output representative of a selected RCP, such sensor output varying between a maximum sensor output level and a minimum sensor output level ("Sensor Output"). The pacemaker provides a pacing ("Pacing Rate") which typically varies as a linear or monotonic function ("f") of the sensor output between a selectable lower pacing rate ("Lower Rate") and upper pacing rate ("Upper Rate"). Function f has a selectable slope (i.e., Pacing Rate change/Sensor Output change) adjustable by means of an external programmer in conjunction with the Lower and Upper Rates. Thus, the Pacing Rate typically provided is equal to the pre-selected Lower Rate plus an increment which is a function of the measured Sensor Output, as follows: EQU Pacing Rate=Lower Rate+f(Sensor Output).
A human's heart rate, however, is normally controlled by a complex set of inputs to the autonomic nervous system. Consequently, no single type of sensor has been found to be entirely satisfactory for controlling rate response functions. Some of the shortcomings of single-sensor, rate responsive pacemakers, for example, can include: (1) long-term sensor instability, such as from degradation; (2) long-term changes in correlation between sensor output and its RCP being measured, due to physiologic changes in the patient, such as biologic/sensor interface changes due to tissue changes; (3) changes in sensor sensitivity; and (4) the need for frequent re-programming to accommodate the foregoing problems, as they are encountered.
Various efforts have consequently been made to develop a multiple-sensor pacemaker which is capable of varying its rate as a function of more than one type of measured RCP. Unfortunately, implementation of such multiple sensor-driven rate response concepts has proven to be very difficult and not entirely satisfactory. In addition to those problems listed above as to single-sensor pacemakers, other problems which are typically encountered include: (1) differences between sensors in long-term stability; (2) differences between sensors in immunity to noise; (3) differences in response time to changing metabolic conditions; (4) differences between sensors in correlation between each sensor output and its RCP being measured; and (5) complex set-up procedures, including the need for frequent re-programming.
Thus, a need exists for a rate response pacemaker which will better accommodate the above-identified problems, preferably in a self-adaptive manner, in the context of a single-sensor or multiple-sensor pacemaker.